Collimated Ballistic Quasiparticle Transport in a Graphene/hBN Superlattice

Moiré patterns in van der Waals heterostructures of graphene and other hexagonal crystals such as boron nitride (hBN) are a readily realizable class of 2D superlattices with electronic properties distinct from those of the parent materials. Transport measurements of highly aligned graphene/hBN superlattices showed a band structure with miniband edges, van Hove singularities, and non-circular cyclotron orbits [M. Lee et al, Science 353, 6307 (2016)]. However, several interesting regimes are obscured by the coexistence of multiple bands in different parts of k-space.
We have recently developed a way to inject collimated beams of electrons into a sheet of graphene: absorptive pinhole collimators, consisting of absorptive sidewalls between a pair of collinear slits, emit beams with angular spread 18 degrees full width at half maximum [A. Barnard et al, Nat. Comm. 8, 15418 (2017)]. This means the collimators populate and/or detect a narrow window of k-states. By filtering orbits in k-space, we can study orbits within specific bands. Here we use collimated beams generated by pinhole collimators in highly aligned graphene/hBN heterostructures to finely probe superlattice band structure and explore possible valley filtering.